Co-located real-time bioaerosol monitoring and measurements of Ice Nucleating Particles (INP) at the rural background station Melpitz

Biological ice nucleating particles (bio-INPs), as a subset of the broader class of biological aerosol particles, are known to be the most efficient INPs at temperatures above -15°C, and several laboratory studies have identified and characterized specific biological ice nucleators (e.g., Hartmann et al., 2025; Wieland et al., 2025). However, in field studies, a clear correlation or even attribution of INPs to specific biological aerosol particles or bioaerosol in general often remains elusive. While several factors contribute to this, one aspect is the lack of measurement techniques that can comprehensively characterize the bioaerosol. In recent years, significant progress has been made in this regard. By combining laser-induced fluorescence (LIF) techniques (fluorescent aerosol particles have typically been used as a proxy for the bioaerosol) with imaging techniques (e.g. holography), we now have instruments capable of identifying and quantifying, for example, pollen and spores of different taxa in situ and in real time. This opens up new possibilities to study the relationship between bioaerosol and INP in the field.

During the 2024 pollen season, we deployed a SwisensPoleno Jupiter (hereafter Poleno) at the ACTRIS-station Melpitz, a rural background station about 40km northeast of Leipzig (Germany). The Poleno is one of the aforementioned instruments that combines UV-induced fluorescence spectroscopy with digital holography (Sauvageat et al. 2020), allowing not only the measurement of bioaerosol concentrations, but also the identification of various taxa (mainly pollen and fungal spores) through an AI-driven classification algorithm. In parallel, a Hirst-type pollen trap was operated and its samples were evaluated by manual pollen and spore counting. These measurements of the pollen trap samples will be used as a reference for the Poleno measurements, as there are few comparative studies in the literature using this relatively new state-of-the-art instrument. In parallel to these measurements, aerosol particles were collected on polycarbonate filters for subsequent off-line INP analysis using droplet freezing array techniques. The INP samples were also heat-treated to determine the fraction of heat-labile, proteinaceous INPs, which is typically used as a lower limit for the amount of bio-INP in a sample.

First results from the comparison of daily mean Pollen concentrations derived with the Poleno (default classification) and the manually evaluated Hirst trap samples show overall good agreement. However, the level of agreement varies depending on the species.
Preliminary results of the INP analysis show generally high INP concentrations (up to 10^-2 #/L at -7.5°C) with indications towards a seasonality, with more ice active samples being more frequent in spring/summer. Correlations of INP concentration (and type) with different bioaerosols (pollen and spores) will be investigated. Additionally, we plan to evaluate the efficacy of short-chained saccharides as an easy-to-measure proxy for pollen concentrations.

Hartmann, S., et al. (2025) Env. Sci. & Tech.
Sauvageat, E., et al. (2020) Atmos. Meas. Tech., 13, 1539–1550
Wieland, F., et al. (2025) Biogeosciences, 22, 103–115